Second generation (2G) mobile communication refers to transmission and reception of voice into digital and is represented by Code Division Multiple Access (CDMA), Global System for Mobile communication (GSM) and the like. General Packet Radio Service (GPRS) was evolved from the GSM. The GPRS is a technology for providing a packet switched data service based on the GSP system.
Third Generation (3G) mobile communication refers to transmission and reception of image and data as well as voice (audio). Third Generation Partnership Project (3GPP) has developed a mobile communication system (i.e., International Mobile Telecommunications (IMT-2000)), and adapted Wideband-CDMA (WCDMA) as Radio Access Technology (RAT). The IMT-200 and, the RAT, for example, the WCDMA are called as Universal Mobile Telecommunication System (UMTS) in Europe. Here, UTRAN is an abbreviation of UMTS Terrestrial Radio Access Network.
Meanwhile, the third generation mobile communication is evolving to the fourth generation (4G) mobile communication.
As the 4G mobile communication technologies, a Long-Term Evolution Network (LTE) whose standardization is being carried on in 3GPP and IEEE 802.16 whose standardization is being carried on in IEEE have been introduced. The LTE uses a term ‘Evolved-UTRAN (E-UTRAN).’
The 4G mobile communication technology has employed Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA). The OFDM uses a plurality of orthogonal subcarriers. The OFDM uses an orthogonal property between Inverse Fast Fourier Transform (IFFT) and Fast Fourier Transform (FFT). A transmitter performs the IFFT for data and transmits the data. A receiver performs the FFT for a received signal to recover original data. The transmitter uses the IFFT for concatenating a plurality of subcarriers, and the receiver uses the corresponding FFT to segment the plurality of subcarriers.
The 3G or 4G mobile communication system continuously attempts to increase a cell capacity in order to support high-capacity services and bidirectional services, such as multimedia contents, streaming and the like.
To increase the cell capacity, an approaching has been proposed to use a high frequency band and reduce a cell radius. If a cell with a short radius, such as a pico cell or the like, is applied, use of a frequency band higher than a frequency used in the conventional cellular system is allowed, thereby enabling transfer of more information. However, such structure requires more base stations to be installed in the same area, which disadvantageously causes a cost increase.
Among approaches of using a small cell and increasing a cell capacity, a femtocell has been recently proposed.
A femtocell refers to installing a base station, which is extremely compact in size and uses low power, in a home/business to provide a small wireless environment. The femtocell is expected to enhance service qualities in response to improving an indoor service available area (coverage) and a capacity, and completely establishing the next generation mobile communication system by providing data services.
3GPP WCDMA and LTE Group are undergoing the standardization of the femtocell called ‘Home eNodeB’ and 3GPP2 is actively undergoing a research for the femtocell.
Different structures are illustrated in FIGS. 1 and 2 in regard of a method for implementing such femtocell in the conventional mobile communication network.
First, FIG. 1 illustrates an exemplary femtocell-based network structure according to the related art.
As illustrated in FIG. 1, the femtocell-based network includes a macro base station (M-BS) having a broad service coverage, and a plurality of femto base stations (f-BSs) installed based on each user.
The f-BS is connected to a femtocell network controller (FNC) via an Internet and controlled thereby, and provides services to a user.
An MS measures signals of neighboring cells and transfers the measured signals to its f-BS. The f-BS recognizes existence of the neighboring cells based on the transferred signals. Also, the f-BSs exchange information via a direct link or an indirect link, such as the FNC. The f-BS and the M-BS exchange information via the FNC and a Radio Network Controller (RNC) or a Mobility Management Entity (MME), which controls the f-BS in a mobile communication network.
FIG. 2 illustrates another exemplary femtocell-based network structure according to the related art.
As illustrated in FIG. 2, the f-BSs exchange information via a direct link or MME, different from FIG. 1. The M-BS and the f-BS exchange information via the MME.
FIG. 3 illustrates an exemplary frame structure used in the femtocell and a macrocell according to the related art.
As illustrated in FIG. 3, a superframe is segmented into four radio frames each having the same size. The superframe may include a superframe header. The superframe header may include essential control information that an MS should acquire upon an initial network entry or handover, and function similar to a Broadcast Channel (BCH) in the LTE technology. The superframe header may be assigned to a first radio frame of a plurality of radio frames constituting a superframe. The number of subframes constituting one frame may be variable to 5, 6, 7 or 8 depending on a bandwidth of a system or a length of a cyclic prefix (CP), and the number of symbols of OFDMA constituting one subframe may also be variable to 5, 6, 7 or 9. FIG. 3 exemplarily illustrates that the length of CP is ⅛ Tb (Tb: Useful OFDMA symbol time) when a bandwidth is 5, 10 or 20 MHz.
The frame structure exemplarily illustrated in FIG. 3 may be applied to a Time Division Duplexing (TDD) or Frequency Division Duplexing (FDD) scheme. In the TDD, an entire frequency band is used for uplink (UL) or downlink (DL) transmission but is divided into UL transmission and DL transmission at a time domain. In the FDD scheme, UL transmission and DL transmission occupy different frequency bands and can be simultaneously performed.
Each subframe may be divided into at least one frequency partition. Each frequency partition may include at least one physical resource unit (PRU). Each frequency partition may include a localized PRU and/or a distributed PRU. Each frequency partition may be used for the purpose of a fractional frequency reuse (FFR), for example.
The PRU indicates a basic physical unit for resource allocation, including N consecutive OFDM symbols and P consecutive subcarriers. A logical resource unit (LRU) is a basic logical unit for distributed resource allocation and localized resource allocation. The LRU includes P*N subcarriers. The LRU includes pilots used in the PRU. Hence, an appropriate number of subcarriers in one LRU may be dependent on the number of allocated pilots.
FIG. 4 illustrates a structure of a synchronous channel (hereinafter, referred to as advanced-preamble (A-preamble)) of IEEE 802.16m (or advanced air interface), which is one of the 4G mobile communication systems. 4 primary or secondary preambles each occupying 1 OFDMA symbol are located within a superframe of 20 ms in size. A superframe header (SFH) by which essential control information is transmitted is transmitted after the secondary preamble symbol. Frequency reuse 1 is applied for Primary Advanced preamble (PA-preamble) transmission, and frequency reuse 3 is applied for Secondary Advanced preamble (SA-preamble) transmission. Therefore, for the SA-preamble, segments are allocated by three types of 1-to-1 mapping according to three types of sector indexes. This exemplary embodiment illustrates that the PA-preamble is located at the second frame, but the present disclosure may not be limited to the case that the PA-preamble is located at the first, third or fourth frame.
FIG. 5 illustrates an exemplary structure showing a femtocell and a macrocell according to the related art, and FIG. 6 illustrates exemplary frames of the femtocell and the macrocell.
As illustrated in FIG. 5, a cell located within the coverage of the M-BS includes a plurality of sectors. The sector means an area defined by a directional antenna of the macrocell. The macrocell, as illustrated, may include three sectors. A segment is defined as a set of PRUs. As illustrated in FIG. 6(a), 24 PRUs within 5 MHz may be divided into three segments, and each segment uses 8 LRUs. In general, one segment is configured within one sector by 1-to-1 mapping. However, the number of segments and the number of sectors may be different from each other, and in this case, mapping between the segment and the sector may depend on a provider's cell planning. The present invention has assumed a typical environment of a communication system having three segments and three sectors.
Each femtocell illustrated in FIG. 5 fixedly uses one sector. However, as illustrated in FIG. 6(b), for the femtocell according to this specification, one sector uses one of the three segments, but the one segment is actively decided by the f-BS.
However, as illustrated in FIG. 5, when a femtocell located in the first sector of the macro cell uses the first segment, which is mapped to the first sector of the macrocell, with the macrocell, an interference with the macrocell may be caused.
In particular, the f-BS is installed at a position fixed by a user, so it is difficult to efficiently manage or avoid the interference with the macrocell.
Similarly, an essential control information channel, like a superframe header is not free from such interference. Since the essential control information channel includes information, such as system information, that all terminals can commonly use, the affection by the interference may cause a very severe obstacle at a terminal's initial network entry or handover.